I would suggest that as the original poster passed the entry tests to get onto the course in the first place, there is little to be gained by doing further tests; be they GAPAN or any other. The initial (I'm assuming Pilapt) tests obviously indicated suitability for training, and a low training risk, or he would not have been offered a place on the course, so I would further suggest that aptitude itself is not the problem.
To echo other posters, the best course of action may well be to find an instructor that you like and get on with, and do some flying.
Good luck!